This disclosure relates to a method for producing a uniform dispersion of particles in a perfluorinated ionomer polymer, for use in electrochemical devices, such as fuel cells.
Fuel cells are electrochemical devices that convert chemical energy in a fuel source to electricity and heat. A fuel cell includes a membrane electrode assembly (MEA), which has an electrolyte membrane sandwiched between two electrodes, an anode catalyst electrode and a cathode catalyst electrode.
A Proton Exchange Membrane Fuel Cell (PEMFC) is one type of fuel cell that is under consideration for high efficiency electricity generation in transportation and stationary applications. A typical MEA in a PEMFC includes a proton-conducting polymer electrolyte membrane and two electrodes, including anode and cathode electrodes. The electrodes typically include nano-particle catalysts, such as platinum or platinum alloys, supported on porous carbon, and perfluorinated proton-conducting polymer ionomer. The electrodes provide a three-phase contact that is necessary for electrochemical reaction, while enabling effective transport of gases (reactant and product), electrons and protons.
Traditional electrode fabrication methods typically include blending perfluorinated polymer ionomer dispersion solution and catalyst nano-particles to form a catalyst ink, and applying the catalyst ink on electrolyte membranes. The techniques for applying catalyst ink on membranes may include screen-printing, solution-spraying or decal-transfer, for example.
The catalyst ink preparation, including selection of the polymer ionomer and blending of the ionomer with the catalyst nano-particle materials, greatly influences the performance of the MEA in a PEMFC. Currently, aqueous perfluorinated sulfonic acid (PFSA) polymer dispersion (i.e. NAFION® dispersion), which has an equivalent weight (EW) between 850 g/mol to 1500 g/mol, has been exclusively used as ionomer material for catalyst ink preparation for PEMFC electrode applications. The EW represents the mass (in grams) of the material that contains one mole of active functional groups, such as —SO3H acid groups in PFSA polymers which function as proton exchange sites in the polymer
Polymer ionomer and catalyst materials are preferably very well mixed. However, as is known in the art, traditional methods for catalyst ink preparation, including but not limited to mechanically dispersing using ultrasonic, high shear mixing and/or ball milling, cannot fully break down the catalyst nano-particle agglomerates in catalyst ink solutions, and therefore, may not produce uniform dispersion of catalyst nano-particles in PFSA ionomer solution/ink used to make the electrodes.
In addition, the EW of PFSA polymer ionomer strongly influences the proton transport ability in the electrodes. The typical EW range of currently commercially available PFSA ionomer dispersion is between 850 g/mol to 1100 g/mol, which cannot provide sufficient proton conductivity in electrodes for high temperature and low humidity PEMFC operating conditions. Linear PFSA ionomer with an EW of less than about 700 g/mol provides superior proton conductivity but is water soluble, and therefore, cannot be effectively applied in electrodes through traditional electrode fabrication methods.